1. Field of the Invention
The present invention relates to compression refrigeration system including a compressor, a heat rejector, an expansion means and a heat absorber connected in a closed circulation circuit that may operate with supercritical high-side pressure, using carbon dioxide or a mixture containing carbon dioxide as the refrigerant in the system.
2. Description of Prior Art
Conventional vapor compression systems reject heat by condensation of the refrigerant at subcritical pressure given by the saturation pressure at the given temperature. These refrigerants are most often selected so that the maximum pressure occurring in the system should be well below the critical pressure of the refrigerant and usually not exceeding a given limit, for example 25 bar.
When using a refrigerant with low critical temperature, for instance CO2, the pressure at heat rejection will have to be supercritical if the temperature of the heat sink is high, for instance higher than the critical temperature of the refrigerant, in order to obtain efficient operation of the system. The cycle of operation will then be transcritical, for instance as known from WO 90/07683.
WO 94/14016 and WO 97/27437 both describe a simple circuit for realizing such a system, in basis comprising a compressor, a heat rejector, an expansion means and an evaporator connected in a closed circuit. CO2 is the preferred refrigerant for both of them due to environmental concerns.
A major drawback for both WO 94/14016 and WO 97/27437 is that very high pressures will occur in the systems during standstill at high ambient temperatures. As explained in WO 97/27437, the pressure will typically be higher than 100 bar at 60° C. This will require a very high design pressure for all the components, resulting in heavy and costly components. Especially this is a drawback in design of hermetic compressors, for which the shell size is dictated by the size of the electrical motor.
WO 94/14016 describes how this can be improved by connecting a separate pressure relieving expansion vessel connected to the low side of the circuit via a valve. The disadvantage of this is that it will increase the cost and complexity of the system.
Yet another drawback of WO 94/14016 and WO 97/27437 is that the charge specifications, respectively 0.55 to 0.7 kg/l and 0.25 to 0.45 kg/l of internal volume of the system will result in too high charge to be optimal for systems for instance operating at lower temperatures of heat absorption and/or using hermetically sealed compressors, having a large gas volume on the low-side of the system.
Another drawback of WO 94/14016 and WO 97/27437 is that they do not take into consideration that the optimal charge of the system will be strongly influenced by the solubility of the refrigerant in the lubricant for systems with lubricated compressors and also by constructive elements of the system.